A magnetic tunnel junction (MTJ) element also referred to as a sensor is a key component of magnetic recording devices. There is a continuous push to increase recording density which requires the sensor to become smaller in order to meet high performance demands of new devices. There are several ways to generate sensors with a smaller CD. One is to reduce the CD by shrinking the mask dimension in the pattern that is printed by a photolithography method into a photoresist mask layer that is coated on a top surface of a MTJ stack. Subsequently, the mask pattern is transferred through the MTJ stack of layers with an etch process to produce a plurality of MTJ elements with a CD similar to that in the photoresist pattern. A second method is to pattern a photoresist mask layer through a photolithography process, and then use a reactive ion etch (RIE) to reduce the photoresist dimension and thereby shrink the dimension (CD) of the sensor that is formed in a subsequent etch process. However, both of these methods have practical limits and cannot reproducibly generate a CD less than about 30 nm which is needed in high performance recording devices.
A MTJ element may be based on a TMR effect wherein a stack of layers has a configuration in which two ferromagnetic layers are separated by a thin non-magnetic dielectric layer. In a GMR sensor, the non-magnetic spacer is typically Cu or another non-magnetic metallic layer. In a sensor, the MTJ element is formed between two shields. A MTJ stack of layers that is subsequently patterned to produce a MTJ element may be formed in a so-called bottom spin valve configuration by sequentially depositing a seed layer, an optional anti-ferromagnetic (AFM) pinning layer, a ferromagnetic “pinned” layer or reference layer, a thin tunnel barrier layer, a ferromagnetic “free” layer, and a capping layer on a substrate. The AFM layer holds the magnetic moment of the pinned layer in a fixed direction. The free layer has a magnetization that is able to rotate from a direction parallel to that of the pinned layer to a direction anti-parallel to the pinned or reference layer and thereby establish two different magnetic states. Alternatively, the MTJ element may have a top spin valve configuration wherein a free layer is formed on a seed layer followed by sequentially forming a tunnel barrier layer, a pinned or reference layer, optional AFM layer, and a capping layer.
Current technology does not enable a solution for achieving a high performance sensor CD or free layer width (FLW) of about 20-25 nm in a reliable manner. Therefore, a new method for fabricating a magnetoresistive sensor is required in order to enable further advances in magnetic recording devices.